Pneumococcal infection
USMLE Step 1Difficulty: easyLast updated:
A 72-year-old man presents with 2 days of fever, pleuritic chest pain, and cough productive of rust-colored sputum. Chest x-ray shows consolidation of the right lower lobe. Sputum Gram stain shows gram-positive, lancet-shaped diplococci. Culture on blood agar grows catalase-negative colonies surrounded by a zone of partial (green) hemolysis.
Which of the following additional laboratory findings is most likely for this organism?
Answer and explanation
Correct answer: C. Sensitivity to optochin and lysis by bile salts
Lobar pneumonia with rust-colored sputum and lancet-shaped, catalase-negative, alpha-hemolytic gram-positive diplococci indicates Streptococcus pneumoniae. The source lists the laboratory features used to identify pneumococci: gram-positive lancet-shaped diplococci, catalase negativity, alpha-hemolysis on blood agar, sensitivity to optochin, and lysis by bile salts. These last two features separate pneumococci from viridans group streptococci, which are also alpha-hemolytic but are optochin-resistant and bile-insoluble. The polysaccharide capsule is the key virulence factor because it helps the organism evade phagocytosis, and it determines the serotype.
Each option explained
- A. Positive coagulase testIncorrect. Coagulase positivity identifies Staphylococcus aureus, which is catalase-positive and grows in clusters.
- B. Resistance to optochin and insolubility in bileIncorrect. Optochin resistance and bile insolubility characterize viridans group streptococci.
- C. Sensitivity to optochin and lysis by bile saltsCorrect. Pneumococci are optochin-sensitive and lysed by bile salts.
- D. Sensitivity to bacitracinIncorrect. Bacitracin sensitivity identifies Streptococcus pyogenes, a beta-hemolytic organism.
- E. Positive CAMP testIncorrect. A positive CAMP test identifies Streptococcus agalactiae (group B), which is beta-hemolytic.
Key point
S. pneumoniae: alpha-hemolytic, catalase-negative, lancet-shaped diplococci that are optochin-sensitive and bile-soluble.
From the source
“In the laboratory, pneumococci are identified by: Gram-positive lancet-shaped diplococci Catalase-negativity Alpha-hemolysis on blood agar Sensitivity to optochin Lysis by bile salts”